1. Field of the Invention
This invention relates to a novel optical element particularly a novel lens of an organic synthetic polymer having a transparent abrasion-resistant coating adhered thereto and to novel and improved methods of making such coated elements.
2. Description of the Prior Art
Heretofore, methods have been proposed for improving the surface characteristics and scratch resistance of solid organic polymers particularly transparent polymeric materials. In U.S. Pat. Nos. 2,404,426 and 2,404,357, methods are disclosed for coating synthetic polymers particularly methylmethacrylate polymers using an ethyl silicate. It is disclosed that the ethyl silicate can be cross-linked using an amine present in combination with moisture during the drying, baking or aging operations.
Abrasion-resistant coatings are also disclosed in U.S. Pat. No. 3,700,487 in which there is claimed a polycarbonate substrate coated with a lightly cross-linked polyvinyl alcohol coating and U.S. Pat. No. 3,484,157 in which there is disclosed an abrasion-resistant plastic transparent optical element having a directly adhering coating comprising a vinyl polymer cross-linked with a dialdeyde cross-linking agent.
Previous attempts to provide an abrasion-resistant coating have failed to provide a sufficiently abrasion-resistant coating which can be applied from a water solution by a dipping operation in which a substantial thickness of coating is applied to the synthetic organic base material. Prior art coatings have been limited to a relatively thin coating of about 1 micron. Those coatings applied by the technique of spin coating, a method well-known to those skilled in the art are especially limited by the thickness of coating which can be applied in one application. The two problems are solved by the methods and compositions of the present invention. Greater abrasion resistance is provided by the use of a relatively thick coating on the synthetic organic polymer substrate. In addition, the methods and composition of the present invention have overcome the discoloration of some prior art coatings when applied in successive layers. The distinct yellow coloration of prior art coatings is objectionable particularly in an optical element.
U.S. Pat. No. 3,986,997 discloses pigment-free coating compositions used to provide abrasion-resistant coatings, particularly for optical materials. This patent does not describe the tie-coat of the present invention nor a tie-coat similar thereto. Example 6 of the patent teaches utilizing a tie-coat prepared by soaking the substrate in 10% solution of potassium hydroxide overnight or priming the substrate with a silane-modified epoxy primer. In addition, although not described in the patent, it is necessary to carefully control the humidity during the coating of the optical elements according to the process of the patent in order to prepare a product having optical properties commercially acceptable.